Primary Keys optional?

OK, just starting a new job with more design than I have done in a while. Looking into things, I have been noticing that many tables have no PK. Some have a unique index, but not all.

When I pointed this out to folks (developers) they shrugged and said "if you need a PK, then create one".

So my questions are:

1. Is it considered acceptable to have a unique index instead of a pk?
2. What are the circumstances when a table might be allowed to exist without any sort of primary key or unique index? (i.e. temp table, static small table, etc)

Thanks In Advance

-- 
Stephen Andert
http://spaces.msn.com/andert-news/
--
http://www.freelists.org/webpage/oracle-l